No group claimed responsibility for Tuesday's attacks, which caused no injuries. No warning was given. No link has been made with the recently discovered Yemen-based mail bomb plot, which used much more powerful devices.

Greece has a vocal anarchist political fringe that opposes most forms of state authority - particularly the police and party democracy - and as well as capitalism and globalization.

In recent decades, small radical anarchist or nihilist groups have staged attacks ranging from nighttime car burnings to bomb and gunfire attacks on symbols of state power and wealth - including the U.S. embassy in Athens.

Merkel was in Belgium when the package arrived in the mailroom of her office.

"It cannot be ruled out that the package contains explosives," her spokesman Steffen Seibert said.

The attacks began Monday when a mail bomb addressed to the Mexican embassy exploded at a delivery service in central Athens, lightly wounding one worker.

Authorities searched surrounding streets and arrested two suspects shortly after the blast. They were carrying mail bombs addressed to French President Nicolas Sarkozy and the Belgian Embassy, along with handguns and bullets in waist pouches. One wore body armor, a wig and a baseball cap.

Police detonated the bombs along with a fourth device found at a delivery company and addressed to the Dutch Embassy.

One of the suspects was wanted in connection with an investigation into a radical anarchist group known as Conspiracy Nuclei of Fire, which has claimed responsibility for a spate of small bomb and arson attacks over the past two years.

The explosions in Greece continuedTuesday with the detonation of a bomb in the courtyard outside a six-story building that is home to the Swiss Embassy.

Swiss Foreign Ministry official Georg Farago said Athens embassy employees regarded the package as suspicious after noticing "traces of metal" on it.

"The package burst into flames when the employees removed the external wrapping of the package. At the same moment, there was an explosion. No one was injured," Farago said.

Soon after, a courier heading for another embassy became suspicious about a package and stopped at Parliament, where police explosives experts detonated a bomb.

Police then found explosive devices at the Bulgarian Embassy and a central Athens courier company - where the German embassy had returned a suspicious package - and set them off in controlled explosions.

A fifth bomb went off on the grounds of the Russian Embassy.

Authorities closed down sections of the capital and checked dozens of potential targets, while all embassies were given additional police security.

Sarkozy said his office took threat seriously and that French authorities were working with Greek police.

"The threat is very serious. We are extremely vigilant and I am following it very closely," Sarkozy said during a joint press conference in London with British Prime Minister David Cameron.

Three people have died in hits claimed by anarchist or far-left groups in Greece since early 2009, and police have arrested more than a dozen suspected militants.

The attacks surged after the December 2008 fatal shooting by police of an Athens teenager, which led to days of rioting throughout Greece.

Much of the unrest harks back to the sharp postwar divide between right and left, which led to a civil war and a seven-year military dictatorship. Although a student uprising succeeded in ending military rule in 1974, there are still tensions between Greece's security establishment and a phalanx of deeply entrenched leftist groups that often protest against globalization and U.S. foreign policy in the Middle East and elsewhere.

"The government condemns in the strongest possible way those who try in vain to terrorize and disturb the public tranquility," government spokesman George Petalotis said. "The police's reaction was excellent, resulting in the arrest of two suspects, and their work will continue in a vigorous manner."

Both suspects were due to appear before an Athens prosecutor later Tuesday.

Police believe the parcels that went off Tuesday were posted the day before.

Parliament speaker Philippos Petsalnikos also condemned what he called "wretched actions."

"No one can terrorize democracy," he said.

---

Associated Press Writers Elena Becatoros and Derek Gatopoulos in Greece, and Colleen Barry in Geneva and Jenny Barchfield in Paris contributed to this report.